1. Field of the Invention
The present invention relates to a preparation process of lactic acid. Lactic acid is widely used for preparing a soft drink, acid food, preserved food and sake in the food industry, or injection liquid and other drugs in the medicine industry. Further, recently, lactic acid is broadly used also in the chemical industry as a material of a biodegradable resin.
2. Description of the Related Art
Lactic acid is usually prepared in industry by a fermentation process and a hydrolysis process of lactonitrile or 2-chloropropionic acid. It has also been known to provide an oxidation process using nitrogen oxide of propylene, oxidation of propylene glycol or propylene oxide, and synthesis by reacting acetaldehyde with carbon monoxide. As to the separation and purification of lactic acid which is prepared by these processes, a general process is to crystallize lactic acid in the form of calcium lactate to separate soluble impurities, successively converting to lactic acid by reacting with sulfuric acid, and separating the calcium ingredient in the form of calcium sulfate crystals.
Further, activated carbon treatment, repeated recrystallization, extraction with an ion-exchange resin or organic solvent, and electrodialysis are carried out in combination in order to enhance purity of lactic acid. In another process, lactic acid is esterified with alcohol such as methanol or ethanol and is distilled to separate impurities. Lactic acid having high purity has been obtained by way of such various purification processes.
However, lactic acid obtained by these processes contains pyruvic acid, reducing sugar and other various impurities due to generation as by-products in the course of preparation or time dependent change by the action of temperature and light. As a result, it has been a problem that applications to the field requiring still higher quality are restricted in view of smell and heat stability of lactic acid.
U.S. Pat. No. 5,310,865 has disclosed a preparation process of high molecular weight polyhydroxycarboxylic acid by direct dehydration condensation of lactic acid or other hydroxycarboxylic acid and a preparation process of a film, filament and molded articles which contain said polyhydroxycarboxylic acid and have an excellent strength.
In the preparation of polylactic acid according to the above technique lactic acid having a relatively high quality is used in order to obtain a high molecular weight polymer having practical strength. However, in such cases, resulting polylactic acid is sometimes colored due to the difference in species or lot of lactic acid used and cannot provide a consistent property.
As to the purification process of trace impurities contained in lactic acid, Japanese Laid-Open Patent SHO 56-65841 has disclosed a process for purifying lactic acid by adding water to an aqueous lactic acid solution and successively conducting distillation. It has been described that the smell of lactic acid can also be improved as a result of removing aldehyde contained in an amount of several tens of ppm and distilling off trace impurities such as methanol and acetic acid at the same time.
However, the process requires addition of a large amount of water for carrying out purification and the cost of concentration and distillation rises. The distilled water has a problem of waste water disposal. Additionally, impurities which are difficult to efficiently remove by the process still remain in lactic acid. For example, pyruvic acid is difficult to remove and thus the process has not always been a satisfactory purification process.
Consequently, the object of the invention is to provide a novel purification process of high quality lactic acid which is free from pyruvic acid and has improved smell and heat stability by removing with a simple procedure pyruvic acid and other trace impurities contained in lactic acid.